Pollination Systems in Paspalum Dilatatum Poir. (Poaceae): An Example of Insect Pollination in a Temperate Grass

The pollination of Paspalum dilatatum was studied in south‐central Oklahoma during the summer of 1979. Pollen was liberated between 0700–0900 hr except on humid mornings (RH ≤ 80%), when there was a delay of 2–3 hr and a reduction in the total air‐borne pollen concentration. A rapid decrease in air‐borne pollen concentration with distance from the source results from: 1) individual pollen grains larger (50–70 μ in diam) than typical wind‐pollinated plants, and 2) some pollen dispersed as clusters of grains. Several floral characters of P. dilatatum led to a hypothesis that this perennial grass may be entomophilous as well as anemophilous. Not only were the pollen grains larger than most other wind‐pollinated taxa but the species produced fewer than 2,500 pollen grains per ovule. Three additional observations corroborate this view: 1) solitary bees (Halictidae) actively collect pollen during the morning, 2) the number of pollen grains per stigma was significantly ( P < 0.001) greater on racemes exposed to both wind and bees than on racemes exposed only to wind, and 3) the combination of bees and wind as pollinators significantly ( P < 0.001) increased seed set compared to wind alone.